for access to the suction valves. Larger pumps are provided with hand holes. General service direct-acting pumps (150 Ib. pressure) up to size 10 by 6 by 10 in. and low service pumps (75 Ib. pressure) up to 16-in. piston are built of the submerged type.
General service pumps, 8-ki. diameter and over, are generally of the straight-way
type, Fig. 17. The suction valves are below the plunger, the discharge valves above. In this way a more nearly straight flow of water through the pump is provided for. They are built as plunger and ring pumps or as piston and sleeve pumps.
The best design of a pump for a pressure up to 300 Ib. is the outside center-packed pump, illustrated in Fig. 18. The plunger can be watched and any cutting readily detected from the outside. Leakage can be stopped while the pump is in operation.
Pressure Pumps.—For high pressures the pump is sectionalized, by casting the barrel separate from the valve chambers. This construction also facilitates the replacement of broken parts. For very high pressures the valves are placed in individual pots carrying one, two or four valves.
Packing.—Two methods are used to prevent leakage through a working fit. One is to fit the two rubbing parts together closely, depending upon the fit to make the joint. This is used in a plunger and ring pump, where these two parts are ground, so as to leave a clearance of not over 0.001 in. The slight leakage through this clearance is insignificant and will remain so for a long time. Besides the pump is always ready to start even after long idleness. The other method is to insert
FIG. 17.—Straightway plunger and ring pump.
FIG. 18.—Outside center-packed pump, twin pattern and rod attached to end of plunger.
some elastic material which by the pressure against the two surfaces will prevent leakage. This material may be some flexible material, such as hemp, cotton, leather, etc. or it may be made of sections of some suitable metal, usually set out